For the production of magnesium hydroxide, precipitation processes are known comprising the addition to a Mg++ containing solution, including sea water, of an alkali such as soda, potash, aqueous ammonia, or lime. In these processes, the alkali is lost in the washing and purification steps of the magnesium hydroxide (Kirk-Othmer, Encyclopaedia of Chemical Technology, third Edition, vol 14, pp 629-631).
U.S. Pat. No. 2,801,155 (Kipper, 1957) describes the production of magnesium hydroxide from dolomites by reacting a dolomite with nitric acid and, in subsequent steps, additionally mixing with dolomite or ammonium gas for producing magnesium hydroxide, which precipitates, and calcium nitrate and/or ammonium nitrate. This process has the disadvantage that the acid used in the leaching of the dolomite, or the alkali used in the precipitation of the magnesium hydroxide are not regenerated.
U.S. Pat. No. 4,937,056 (Kirk et al., 1990) describes a process for the production of magnesium hydroxide from solids containing magnesium in the form of carbonate, hydroxide or oxide, with an impurities content of at least 5%, by leaching with a SO2 solution. This process has the disadvantage that, in order to reduce impurities, gradual pH increments are made to precipitate such impurities selectively. The pH increments are of 0.5 units and, consequently, very sensitive controls are required in operating the process.
U.S. Pat. No. 6,214,313 (Berisko et al., 2001) discloses a process for the production of magnesium hydroxide from a stream originated in the desulphurization of fuel gases, which has the disadvantage that the magnesium sulphite of the solution must first be converted to sulphate by the addition of hydrogen peroxide and then, the magnesium sulphate is precipitated with sodium hydroxide.
In some of the processes commonly used for the production of magnesium hydroxide, the starting material used is a solid compound which, upon reacting, produces impurities such as calcium sulphate, carbonate, hydroxide or oxide, all of which can be precipitated and dragged with the product, resulting in a low purity product. These processes are generally energy intensive (demanding high electrical energy and fuel consumption) and the purity of the magnesium hydroxide produced depends directly on the purity of the starting materials used in the process.
As it can be observed, the above-mentioned processes do not guaranty the production of magnesium hydroxide of a purity enabling its use in demanding technical areas, such as the manufacture of pharmaceutical and food products, using starting materials not necessarily of high purity. Consequently, complex purification operations are required, such as the selective dissolution of undesired non-soluble substances. For instance, in U.S. Pat. No. 5,626,825 (Verri, 1997) a purification process of magnesium hydroxide is described by means of the addition of a chelating agent which traps the impurities contained in a magnesium hydroxide slurry, thus obtaining a purity of at least 98%. It should be noted that this process is intended for purifying a magnesium hydroxide product already produced. U.S. Pat. No. 4,693,872 (Nakaya et al., 1987) discloses a process for the production of high purity magnesium hydroxide starting from an impure magnesium hydroxide which, in a first step, is dissolved in a stream of calcium chloride resulting from the regeneration of ammonia in a subsequent step, and by the addition of carbon dioxide gas. This process has the disadvantages of requiring the handling of two gaseous streams and the requirement of hermetic equipment.
As can be observed from the above references, purification processes are normally carried out as an additional step in the production of magnesium hydroxide of low purity to obtain the desired purity degree.
The physical and chemical properties of byproducts or impurities resulting from the production of the magnesium hydroxide can be used advantageously in other technical fields such as water purification, as auxiliary filtering media for improving the quality of processed liquids, since they retain heavy metals, suspended particles and organic materials. Surprisingly, no advantage has been taken from these properties for the purification of the magnesium hydroxide during its production.